The remarkable immunostimulatory properties of dendritic cells (“DC”) reside in their ability to transport antigens from peripheral tissues to lymphoid organs where they present these antigens to T-cells in an optimal costimulatory context (1). To achieve this complex sequence of events, DC exist in different functional stages. Immature DC behave as sentinels in peripheral tissues where they efficiently capture antigens. Upon pathogen invasion, induction of protective T-cell responses require the activation of immature DC into mature immunostimulatory cells. DC activation is triggered in inflamed tissues by cytokines such as IL-1 and TNF-a and by bacterial components such as lipopolysaccharide (LPS) (2, 3). Activated DC migrate to T-cell areas in the lymph nodes while upregulating their costimulatory capacities and optimizing their antigen presenting functions. Upon interaction with antigen-specific T-cells, DC activation is further completed through engagement of the receptor-ligand (1) pair CD40-CD40L, leading to the production of IL-12 (4, 5, 6), a key cytokine for T helper (Th) type 1 and cytotoxic T lymphocyte (CTL) priming (7).
Antigen Presenting Cell (APC) activation through CD40-CD40L interactions represents an important immunoregulatory step for the establishment of protective T-cell immunity against pathogens and tumors (8, 9, 10). This process also plays a key role in the onset of destructive T-cell-mediated disorders such as autoimmune diseases, allograft rejection and graft versus host disease (11, 12, 13). The current treatment of these disorders largely relies on the administration of glucocorticoids (the abbreviation “GC” is used herein for the terms “glucocorticoids” and “glucocorticoid”), which exert potent anti-inflammatory and immunosuppressive effects. Because GC negatively interfere with many aspects of T-cell activation, such as IL-2-driven proliferation and inflammatory cytokine production (reviewed in 14), activated T-cells have long been considered as the main targets for GC action. Several lines of evidence now suggest a role for DC in GC-induced immune suppression. Moser et al. (15) found that GC prevented the spontaneous activation of murine DC thereby decreasing their T-cell stimulatory potential. Kitajima et al. (16) showed that GC could hamper the T-cell-mediated activation of a murine DC line. Viera et al. reported that human DC exposed to GC were poor producers of IL-12 upon LPS stimulation (17). These findings only concern loss of typical DC features and, therefore, favor a simple inhibitory role of GC on DC activation. A more complex immunoregulatory action on the DC system has not been considered.
The present invention resulted from a detailed analysis of the impact of GC on the CD40-mediated activation of monocyte-derived DC. These DC develop after culture with GM-CSF and IL-4 (2, 18) or after transmigration through endothelial cells (19) and are known to mature into the most potent human Th1-type-inducing APC upon CD40 ligation (5, 20). Moreover, these APC can easily be generated in large numbers and are thereby the cells of choice for DC-based modulation of T-cell immunity (21, 22). In contrast to previous studies, the present invention shows that GC, such as dexamethasone (DEX), do not merely prohibit DC activation but converts CD40 ligation on human monocyte-derived DC and is transformed into an alternative activation pathway. DEX profoundly affects the CD40-dependent maturation of human monocyte-derived DC, not only by preventing the upregulation of costimulatory adhesion and MHC surface molecules, but also by causing these cells to secrete the anti-inflammatory mediator IL-10 instead of the Th1 stimulatory cytokine IL-12. In agreement with these phenotypic and functional changes, DC triggered through CD40 in the presence of DEX are poor stimulators of Th1-type responses. Most importantly, the present invention shows that such DC are able to induce a state of hyporesponsiveness in Th1 cells, indicating that these cells are capable of active suppression of Th1-type immunity.